In one known apparatus of this type, the power supply is manually adjusted when the apparatus is installed on site, in order to compensate for the effect of said leakage currents.
However, this adjustment is difficult to perform and is thus a source of inaccuracy in measurement, in addition to requiring action by qualified personel.
Preferred embodiments of the present invention thus provide measuring apparatus of the above type in which there is no need for adjustment when the apparatus is installed on site, in spite of the fact that the leakage currents depend on the length of the electrical link and that this length is unknown when the measuring apparatus is manufactured.
Measuring apparatus is also known in which the sensor acts by varying the value of an electrical resistance, and in which the potential drop due to the link itself is measured in order to compensate the measured result.
This apparatus not only requires an additional conductor for the purpose of measuring said potential drop in the link, but also is only capable of compensating for interference when the electrical characteristic (resistance in this case) which is modified by the quantity to measured is the same as the characteristic giving rise to the interference.
Preferred embodiments of the present invention thus also provide measuring apparatus in which the measurement is based on variation of a first electrical characteristic (eg. inductance) and which are capable of compensating for interference due to a second electrical characteristic (eg. resistance).